Pull-in and hanging up of flexible risers to for example an FPSO (a vessel for floating production, storage and offloading) is a comprehensive and time consuming operation.
At arrival to the field the risers will normally have been laid down on the sea bed. The risers are pulled up to the vessel by use of a winch, optionally by assistance of an installation vessel. In the case with a FPSO it is usual practice to use a turret, through which the risers are pulled in through dedicated guiding pipes by use of a winch. Preferably a guiding sheave is used, which can be positioned along a runway on the turret, and a wire arranged between the winch, over the guiding sheave and to the upper end of the flexible riser. There are two dominating methods for hanging up of the risers. By the first method, the risers are brought up through a guiding pipe in the turret to deck level, where it is hanged up on a flange connection or similar, and can be connected to other equipment, for example a process plant. By the other method both the riser and the bend restrictor is brought up to the lower edge of the turret, where a connection to a dedicated coupling at the lower end of a stiff pipe extending through the guiding pipe in the turret is made. By both methods a bend restrictor is fastened to the lower end of the guiding pipe. The first method has advantage by the hanging up of the riser being made at deck level of the vessel, compared to the other method for which the hanging up of both the riser and the bend restrictor is made at the lower end of the guiding pipe. All work under the turret and hull is difficult, and is usually undertaken in a difficult working position under water, which is the case also for the first method, more specifically, fastening of the bend restrictor. However, coupling mechanisms exist to facilitate the assembly, but the mechanisms take up a substantial space and are susceptible to damage. The methods results in two working operations, of which the hanging up of risers, and mounting of bend restrictors, are undertaken successively.
The bend restrictor transfers large forces from the riser to the fastening in the lower end of a stiff guiding pipe. The torques and stresses become very high compared to the capacity of flexible risers.
The constructions according to the above-mentioned methods results in some difficult working operations, they are complicated and clumpy, and are not feasible for hanging up by working operations undertaken only at deck level, and results in a high mechanical load level.
There is a demand for a construction of an upper termination for flexible pipes, which results in that all work can be undertaken at a deck level, with less torque and stresses on the flexible pipe as hanged up, with larger extent of pre-assembly, with less and simpler working operations to hang up the riser, with more compact construction to facilitate pull-in, and with construction such that the forces are taken up in the longitudinal direction of the pipe with respect to the fastening means at deck level.